The present invention relates generally to a system and method for measuring capacitance. More particularly, the present invention relates to a system and method for measuring capacitance between a probe and a semiconductor sample.
The present invention may be useful in the field of scanning capacitance microscopy (SCM). In a typical embodiment of SCM, a nanometer scale-conducting tip is scanned across a sample surface, and a capacitance detector measures variations in the probe-sample capacitance C. To date, some of the most common applications of SCM have been semiconductor characterization including dopant profiling, device characterization, and surface defect characterization. A common thread in these measurements is that the samples exhibit voltage dependent capacitance due to a voltage-dependent space charge layer in the semiconductor substrate. These implementations of SCM generally do not measure C directly. Rather, they typically measure dC/dV by varying the probe-sample voltage V at frequencies greater than 10 kHz. In other words, existing SCM circuits are typically not adapted for calibrated low frequency measurements of absolute capacitance.
However, not all systems of interest contain a semiconductor space-charge layer that exhibits voltage dependent capacitance. This is particularly true for thin dielectric films on highly conducting (metallic) substrates. In this case, it is necessary to measure the capacitance directly rather than measuring dC/dV. Examples of such thin-film systems include perfluoropolyether compounds that are of fundamental importance in the lubrication of diskdrive and micro-electromechanical machine systems (MEMS). Previously, a macroscopic form of “direct” SCM has been used to used to monitor thin, perfluoropolyether lubricant films with mm-scale lateral resolution by applying an AC voltage and measuring the resulting displacement current.
Despite the many benefits of known embodiments of SCM, there are still many needs in the field of capacitance measurement. For instance, a need exists for a SCM technique that can achieve a lateral resolution of <500 nm. A need also exists for a system and method for calibrated low frequency measurements of absolute capacitance of a probe-sample junction. Also, a need exists for determining the stray capacitance of a probe-sample junction. A further need exists for factoring out the stray capacitance of a probe-sample junction when measuring the tip-sample capacitance.